A lake-wide approach for large lake zooplankton monitoring: Results from the 2006-2016 Lake Superior Cooperative Science and Monitoring Initiative surveys.

Whole-lake surveys of Lake Superior were completed during late summer in 2006, 2011, and 2016 to assess lower food web conditions under the Cooperative Science and Monitoring Initiative (CSMI). These surveys used a spatially stratified probability approach based on depth to assess food web conditions within different depth zones. We evaluated differences in crustacean zooplankton biomass, rotifer density, and the community structure of both groups in nearshore (<30 m), midshore (30-100 m), and offshore (>100 m) depth zones and investigated changes in these parameters within zones over time. Although nearshore crustacean biomasses and rotifer densities were highly variable, the depth zones differed from each other based on these parameters and should be considered separately. Crustacean biomass, community structure, and vertical position were consistent over time across depth zones. The differences that did occur were within the range of known annual variability. Total rotifer densities were lower in 2016 than in 2006 in all depth zones but the genera that contributed to the lower values were not the same across zones. Further studies are needed to know whether these differences reflect annual variability or long-term trends. Finally, we show how the depth zones used in this study can facilitate comparisons between monitoring programs. This is important because most zooplankton studies are limited to certain depth zones and changes in zooplankton parameters may not occur uniformly across zones. The high variability in nearshore zooplankton parameters suggests that additional research may be needed to effectively track changes there.